It is well known that in the presence of radiofrequency (RF) irradiation, spin magnetization has different relaxation properties from T1 and T2. In the rotating reference frame, the components of the magnetization parallel and perpendicular to the effective field have characteristic relaxation times T1ρ and T2ρ, respectively. Like T1, T1ρ varies, or disperses, with field strength because of energy exchange with the lattice. In MR imaging, T1ρ contrast is useful because only frequency components of the lattice that are equivalent to the amplitude of the RF field can cause relaxation in the rotating frame. These frequency components are typical of slow exchange, such as proton water exchange with hydroxyl and amide functional groups, slow rotation, static dipolar or quadrupolar interactions.
There are a great number of T1ρ pulse sequences for imaging that all require both magnetization preparation to sensitize the signal to relaxation and long delay times to restore equilibrium. This paradigm is inherently time inefficient. Instead, it might be desirable to continuously acquire the T1ρ-weighted signal in the steady-state. Certainly, a few T1ρ sequences employ very short delay times, and, therefore, a steady-state is formed (see Borthakur, et al., “Three-dimensional T1 rho-weighted MRI at 1.5 Tesla,” J. Magn. Reson. Imag. 2003, 17(6): 730-736). This technique is rarely used in practice because of the significant signal loss incurred when equilibrium is not fully restored and, because of specific absorption rate (SAR) constraints, the minimum scan time is usually much greater than a magnetization prepared multi-acquisition scheme that allows full recovery of longitudinal magnetization.
Steady-states in MRI are ubiquitous. Likely the most well known steady-state contrast is the short TR, low flip angle, spoiled gradient echo, which produces T1 contrast. Equally well known is the short TR balanced steady-state free precession (bSSFP) sequence, which produces a T2/T1 contrast. Unfortunately, it is not clear how to establish a steady-state T1ρ contrast with significant signal, since, on-resonance, the rotating frame thermal polarization is nearly zero with RF fields appropriate for clinical use. On the other hand, the steady-state of an off-resonance spin lock can be significant, but the problem remains to deliver an off-resonance spin locking RF pulse train interrupted briefly for a short period of data acquisition.